#Error after lookup function returning a calculated field

Following is my expression for a calculated field in Dataset3:

CDec(Fields!WGA_Premie_WG_sal.Value + Fields!WGA_Premie_WG_pens.Value + Fields!WGA_Premie_WG_wia.Value + Fields!WGA_Premie_WG_anw.Value + Fields!WGA_Premie_WG_inh.Value)

When I do a lookup tot this field in my report

=CDec(code.AddTotalWgaPremieWG(CDec(Lookup(Fields!Medewerkercode.Value, Fields!Medewerkercode.Value, Fields!WGA_Premie_WG.Value, "DataSet3")),Fields!MedewerkerPremieGroep.Value))

The result is that first page of the report shows normal decimal values, but from the second page I get #Error in stead of decimal values.

Adding CDec conversion does not help. When I try this with individual query fields from dataset3 everything goes well.

Please help or advise